Substituted 2,5-dimethylpyrroles

ABSTRACT

N-substituted 2,5-dimethylpyrroles and a method for their preparation. The method requires the reaction of a 2,5-hexadione such as acetonylacetone with a urethane or a phosphoroamidate. The reaction is carried out at elevated temperatures, preferably in a water-immiscible solvent.

This is a division of application Ser. No. 458,602 filed Jan. 17, 1983,now abandoned.

This invention relates as indicated to substituted 2,5-dimethylpyrrolesand more particularly to a process for the preparation of suchcompounds.

BACKGROUND OF THE INVENTION

The substituted pyrroles of this invention are useful as intermediatesin the syntheses of polymer additives which in turn are effective toprotect various polymers from deterioration caused by exposure toultraviolet light. The substituted pyrroles of the invention can bereacted with acetylene dicarboxylic acid, in a Diels-Alder reaction, forexample, to form a much larger, less volatile compound which iseffective to inhibit the deterioration of polypropylene upon exposure toultraviolet light. This Diels-Alder product, containing an olefinicdouble bond, may be hydrogenated to yield a product which is an evenmore effective ultraviolet light stabilizer. Other active dienophilesmay of course also be used for this purpose and illustrative examplesinclude dialkyl maleates, alkyl acrylates and methacrylates, diethylacetylene dicarboxylate, propargyl alcohol and butynediol.

The effectiveness of these Diels-Alder reaction products as lightstabilizers is believed to be due to the hindered amide group.

Also, the acyl and phosphoryl groups may be removed (by hydrolysis) fromthese Diels-Alder condensation products and the resulting hinderedamines likewise are effective ultraviolet light stabilizers in polymercompositions.

While a wide variety of polymers are benefitted by the protective actionof these Diels-Alder products, olefin polymers are especiallybenefitted. Polypropylene, in particular, is susceptible tostabilization by the addition of a small proportion of such an additive.

The condensation of gamma-diketones such as hexane-2,5-dione, i.e.,acetonylacetone, with primary amines to form pyrroles, is shown at page77 of "The Chemistry of Pyrroles" by Jones et al., Academic Press(1977). The reaction is referred to as the Paal-Knorr condensation. Itappears that the condensation reactions were carried out in aqueoussystems because there is a considerable discussion about the optimum pHat which the reaction may be carried out. Moreover, it is stated that2,5-dimethylpyrrole may be prepared from the reaction ofhexane-2,5-dione and formamide; such a result was obtained in an aqueousenvironment.

SUMMARY OF THE INVENTION

The invention here is a substituted 2,5-dimethylpyrrole having themolecular structure ##STR1## where R is alkyl or aralkyl and contains1-19 carbon atoms, A is carbon or phosphorus, n is 1 or 2, and R' ishydrogen or carboalkoxy having 2-13 carbon atoms. The invention alsoincludes a process for preparing such substituted 2,5-dimethylpyrrolescomprising reacting acetonylacetone or a 3-carboalkoxyacetonylecetonewith a urethane or phosphoramidate having the molecular structures,respectively, ##STR2## wherein R is alkyl or aralkyl of 1-19 carbonatoms. The process is illustrated by the following equations: ##STR3##

DETAILED DESCRIPTION OF THE INVENTION

R in the above structure may, as indicated, be alkyl or aryl. It maycontain 1-19 carbon atoms. Illustrative examples of such R groupsinclude methyl, ethyl, n-propyl, n-heptyl, n-nonyl, n-decyl,n-octadecyl, benzyl, beta-phenylethyl, etc.

Specific illustrative urethanes include aralkyl carbamates such asbenzyl carbamate, alkyl carbamates such as methyl carbamate, ethylcarbamate and the like, as well as mixtures thereof, while thephosporamidates useful for the purposes of this invention includedialkylphosphoramidates such as diethylphosphoramidate,di-n-hexylphosphoramidate, etc.

It will be noted that the amide group of the composition of theinvention is hindered by the two methyl groups in the 2- and 5-positions. It is believed that such hindrance is a factor in the notableeffectiveness as light inhibitors of the Diels-Alder products which maybe prepared from these substituted pyrroles.

The process of the invention is carried out in an anhydrous system. Insome instances, to insure the substantial absence of water, it isadvisable to heat, at reflux temperature, a solution of the amide in awater-immiscible solvent such as toluene, collecting any water in aDean-Stark trap. Then when no more water is thus collected,acetonylacetone is added and the whole is heated until the reaction iscomplete.

The process requires the reaction of one mol of amide per one mol ofacetonylacetone and, generally, these are the proportions of reactantsthat should be used for a most efficient reaction. The use of asubstantial excess of either reactant merely results in the loss of theexcessive amount of that reactant.

A catalyst ordinarily is used. Acidic catalysts are preferred.Illustrative examples of suitable acidic catalysts includep-toluenesulfonic acid, methanesulfonic acid, sulfonic acid, phosphoricacid, cationic resins such as sulfonated copolymers of butadiene andstyrene, dilauryl phosphoric acid and the like.

It is desirable to use a solvent. Among other reasons, it faciliatesremoval of water, as it is formed, from the reaction mixture, viz., bymeans of a Dean-Stark trap. Water-insoluble solvents should be used.Toluene, benzene, xylene, heptane, tetrachloroethane and chlorobenzeneare illustrative. The boiling point of the solvent may range from about75° C. to about 200° C., although a narrower range is preferred so as topermit easy removal (at a lower temperature) of the solvent from theproduct mixture, i.e., from about 100° C. to about 140° C.

The process is carried out quite simply; the reaction mixture is heatedat a temperature within the range of from about 75° C. to about 200° C.,usually at the reflux temperature of the solvent. Water is removed fromthe product mixture as it is formed and when no more water is formed thereaction is halted. The N-acyl-2,5-dimethylpyrrole product is isolatedby distillation. The distillate usually comprises a mixture of thedesired pyrrole and a small proportion of unreacted acetonylacetone.This latter can be removed by extraction with a solvent such as heptane;i.e., one which dissolves acetonylacetone more readily than thesubstituted pyrrole.

EXAMPLE 1

A solution of 55 g (0.364 mol) of benzyl carbamate, 42 ml. (40.4g.-0.372 mol) of acetonylacetone and 0.25 g. of p-toluenesulfonic acidin 100 ml. of toluene is heated at reflux temperature for 7.5 hours,collecting evolved water in a Dean-Stark trap. A total of 13.5 ml. (0.75mol) of water is thus collected. The product mixture is concentrated byheating to 130° C./5 mm. The residue is taken up in 100 ml. of heptaneand filtered. The filtrate is concentrated to a purple solid, M.P.62°-65° C., the desired carbobenzyloxy-2,5-dimethylpyrrole.

EXAMPLE 2

A solution of 10 g. (0.0653 mol) of diethyl phosphoramidate, 7.5 ml.(7.3 g.-0.0632 mol) of acetonylacetone and 0.1 g. of p-toluenesulfonicacid in 50 ml. of benzene is heated at reflux temperature. Evolved wateris collected in a Dean-Stark trap. The residue is distilled at90°-92°/0.1 mm. The distillate (shown below) ##STR4## weighs 14.5 g.

EXAMPLE 3

A solution of 28. g. (0.246 mol) of acetonylacetone and 22 g. (0.247mol) of ethyl carbamate in 50 ml. of ethanol is heated at refluxtemperature for 1.5 hours. Gas chromatographic analysis indicates theformation of a small proportion of desiredN-carboethoxy-2,5-dimethylpyrrole. The mixture is freed of ethanol bystripping, then heated at 150°-180° C. for six hours and distilled.

The distillate is shown by gas chromatographic analysis to be a mixtureof starting materials and desired product. This mixture is dissolved in50 ml. of carbon tetrachloride, 0.1 g. of p-toluenesulfonic acid isadded, and the whole is heated at reflux temperature for ten hoursduring which period four ml. of water is collected in a Dean-Stark trap.The product mixture is distilled yielding two principal fractions ofwhich the first fraction, weighing 30 g., was shown to contain mostlydesired product and the second fraction, weighing 7 g., was shown to besubstantially pure desired product, i.e.,n-carboethyloxy-2,5-dimethylpyrrole.

It is apparent from this example that while the process of the inventioncan be carried out without an acidic catalyst, and without awater-immiscible solvent, it is carried out more efficiently when thesetwo conditions obtain.

EXAMPLE 4

A solution of 70 g. (0.458 mol) of diethyl phosphoramidate, 52.5 ml(51.1 g.-0.449 mol) of acetonylacetone and 0.1 g. of p-toluensulfonicacid in 200 ml. of benzene is heated at reflux temperature for severalhours until a total of 15 ml. of water had been collected in aDean-Stark trap. The product mixture is distilled into three fractionsweighing 8.5 g. (B.P., <80° C./0.1 mm.), 71 g. (B.P. 80° C./0.1 mm.) and15 g. (B.P. 82°-85° C. mm.), respectively. The middle fraction is shownby infrared analysis to be substantially pure substituted pyrrole of thefollowing structure: ##STR5##

EXAMPLE 5

A solution of 28.0 g. (0.151 mol) of 3-carboethoxy-2,5-hexanediene, 13.4g. (0.151 mol) of ethyl carbamate and 0.1 g. of p-toluenesulfonic acidin 100 ml. of heptane is heated at reflux temperature until a total of5.0 g. of water is collected in a Dean-Stark trap. The product mixtureis distilled yielding 15 g. of a fraction boiling at 115°-120° C./0.1mm., n_(D) ²⁰, 1.4897, and shown by gas chromatographic analysis to bethe compound shown below: ##STR6##

EXAMPLE 6

A solution of 114 g. (1.54 mols) of methyl carbamate, 180 ml. (175.3g.-1.54 mols) of acetonylacetone and 0.1 g. of p-toluenesulfonic acid in300 ml. of benzene is heated at reflux temperature until a total of 52ml. of water is collected (in a Dean-Stark trap). The product mixture isstripped, then distilled yielding 208.5 of distillate which isidentified as the desired N-carbomethoxy-2,5-dimethylpyrrole by means ofinfrared and gas chromatographic analysis.

All parts and percentages herein are by weight unless otherwiseexpressly stated.

We claim:
 1. A substituted 2,5-dimethylpyrrole having the molecularstructure: ##STR7## wherein R is alkyl or aralkyl and contains 1-19carbon atoms, and R¹ is hydrogen or carboalkoxy having 2-5 carbon atoms.2. The substituted 2,5-dimethylpyrrole of claim 1 wherein R is alkyl. 3.The substituted 2,5-dimethylpyrrole of claim 1 wherein R is alkyl of 1-4carbon atoms.
 4. The substituted 2,5-dimethylpyrrole of claim 1 whereinR¹ is hydrogen.
 5. A process for preparing substituted2,5-dimethylpyrroles having the molecular structure: ##STR8## wherein Ris alkyl or aralkyl and contains 1-19 carbon atoms, R' is hydrogen orcarboalkoxy having 2-5 carbon atoms, said process comprising reactingacetonylacetone or a 3-carboalkoxy acetonylacetone with a urethanehaving the molecular structure:

    ROCONH.sub.2

wherein R is alkyl or aralkyl of 1-19 carbon atoms in a water-immisciblesolvent at a temperature of between 75° and 200° C. in the presence ofan acidic catalyst, removing water and isolating the said substituted2,5-dimethylpyrrole.
 6. The process of claim 5 wherein R is alkyl. 7.The substituted 2,5-dimethylpyrrole of claim 1 wherein R is selectedfrom the group consisting of benzyl, ethyl and methyl.
 8. The process ofclaim 5 wherein R is selected from the group consisting of benzyl, ethyland methyl.